Fusion surgery considerations
In addition to the primary reasons for failed back
surgery discussed on the prior page, there are
several reasons why a spine fusion surgery might fail
to alleviate a patient’s back pain.
Failure to fuse after spine surgery
When the fusion is for back pain and/or spinal instability, there is a correlation
(although weak) between obtaining a solid fusion and having a better result of
the spine surgery. If a solid fusion is not obtained through the spine surgery,
but the hardware is intact and there is still good stability to the spine, the
patient may still have good pain relief with the spine surgery. In many cases,
achieving spinal stability alone is more important than obtaining a solid fusion
from the spine surgery.
On postoperative imaging studies it is often very difficult
to tell if a patient’s spine has fused, and it
can be even harder to determine if a further fusion
surgery is necessary. In general, it takes at least
three months to get a solid fusion, and it can take
up to a year after the spine surgery. For this reason,
most surgeons will not consider further spine surgery
if the healing time has been less than one year. Only
in cases where there has been breakage of the hardware
and there is obvious failure of the spinal construct
would back surgery be considered sooner.
Implant failure in spine surgery
An instrumented fusion can fail if there is not enough
support to hold the spine while it is fusing. Therefore,
spinal hardware (e.g. pedicle screws) may be used
as an internal splint to hold the spine while it
fuses after spine surgery. However, like any other
metal it can fatigue and break (sort of like when
one bends a paper clip repeatedly). In very unstable
spines, it is therefore a race between the spine
fusing (and the patient’s bone then providing
support for the spine), and the metal failing.
Metal failure (also called hardware failure, implant
failure), especially early in the postoperative course
after spine surgery, is an indicator of continued gross
spinal instability. The larger a patient is and the
more segments that are fused, the higher the likelihood
of implant failure. Implant failure following spine
surgery should be very uncommon in normal sized individuals
with a one level fusion.
Transfer lesion to another level after a spine
fusion
A patient may experience recurrent pain many years after a spine fusion surgery.
This can happen because the level above a segment that has been successfully
fused can breakdown and become a pain generator.
- This degeneration is most likely to happen after
a two-level fusion (e.g. a fusion for L4-L5 and L5-S1
levels) and in a young patient (in the 30-50 year
old age range).
- It is much less likely to happen if only the L5-S1
level is fused, as this segment typically does not
have much motion and fusing this level does not change
the mechanics in the spine all that much.
- Most of the motion in the spine is at the L4-L5
level, and to a lesser extent at L3-L4. When the L4-L5
level is included in the spine fusion it transfers
a lot of stress to L3-L4. This does not present as
much of a problem for elderly patients, since they
tend to not be as active nor do they have the fusion
for as many years.
|
